With the advent of widespread use of cellular telephones and the corresponding growth of wireless subscribers for using such telephones, a need has arisen for increasing a network's capacity within an existing communication infrastructure. Before describing how this need has been addressed, however, a general structure of a communication system will be described.
FIG. 1 illustrates a basic cellular network, which comprises a plurality of radio cells, respectively labeled A-G. Such radio cells are defined by a base station (BS) at their center and are distributed evenly in clusters. An area covered by each of the plurality of radio cells is indicated by the dashed lines surrounding the radio cell. A number of communication technologies are currently used by the cellular and PCS service providers, such as AMPS (FDMA), TDMA, and CDMA. Both AMPS and TDMA do not use a same frequency in the adjacent cells. In these technologies, the same frequency is reused in every "n" cells, which are clustered together. This concept is referred to as a principle of a "repeat cell", and alternatively, as a "frequency reuse factor".
In CDMA technology, the same frequency is used in all cells within the network. Mobile users move from one cell to another cell through a process known as "soft" handoff without switching a frequency at which the mobile user is operating. Where another frequency is used within the CDMA network, the mobile user may handoff to another frequency by switching its frequency to the new channel frequency. This process is referred to as "hard" handoff. Since the same frequency is used throughout the network, the mobile users of the surrounding cells will generate interference in the cells with which they are communicating. The ratio of the in-cell interference, to the total interference as seen by the base station of each cell, is referred to as the "frequency reuse factor". Note that this terminology is specific to CDMA technology and is modified from terms typically used with TDMA and AMPS technology.
FIG. 2 illustrates the basic components of a wireless communication network architecture. As is illustrated in FIG. 2, the infrastructure of a communication network generally includes multiple mobile switching centers (MSCs) which provide control, tracking, and data about mobile users within a predetermined area. FIG. 2 illustrates a distributed home location register wireless network architecture which is typically utilized to establish an infrastructure for a wireless communications network. In such an architecture, a home MSC tracks and determines where a mobile user is currently registered. A mobile user is registered on a home location register (HLR) within a home mobile switching center (MSC). When the mobile user travels away from their home MSC, the mobile user is detected by a second mobile switching center. The second mobile switching center then provides information to the home MSC and the home MSC provides control information to register the location of the mobile user on a visiting location register (VLR) within the visited MSC. An illustrative example of operation will be provided below.
In FIG. 2, if mobile user 222 is registered on HLR 208 of MSC 210, MSC 210 is said to be a home MSC of mobile user 222. When mobile user 222 travels from home MSC 210, mobile user 222 will be detected by a second MSC 212 via base station B 220. MSC 212 provides information to MSC 210 indicating that mobile user 222 is within an area serviced by MSC 212 and base station B 220. MSC 210 subsequently registers a location of mobile user 222 on VLR 214 within MSC 212.
Subsequently, when a call is received by MSC 212 from public switched telephone network (PSTN) 204, the current visited MSC 212 determines whether the mobile user to whom the call is directed is registered in HLR 216 or VLR 214. Since mobile user 222 is registered in VLR 214 of MSC 212, MSC 212 communicates information with the mobile user's home MSC 210 of the call is directed to mobile user 222. Through this method, home MSC 210 is able to transfer the call to a current MSC 212 so that mobile user 222 receives the call even when they are not within their own home MSC.
As more and more mobile users, such as mobile user 222, utilize a cell within a wireless communication network, a higher demand for service is created. To meet this demand for service, an increased number of communication channels is required. However, the number of channels which may be utilized is limited by an available bandwidth of frequencies within a cell. As the right to use certain frequency bandwidths is a limited commercial commodity, the costs associated with obtaining such frequencies are not minimal. Therefore, the overhead associated with obtaining increased frequency bandwidth is often too substantial to be commercially viable and implemented in wireless communication networks.
Additionally, in implementations in which communication channels are limited, a service provider may opt to implement smaller communication cells or to increase a number of sectors of the cells, in those areas where a demand for service is high. The implementation of small cells is referred to as "cell splitting." When cell splitting is implemented, a capacity of a system may be increased by reducing a size of the cell so that the total number of channels available per unit area is increased. "Cell splitting" is achieved by placing base stations at specific points in a cellular pattern, typically reducing a cell area by a factor of 3 or 4. By repeatedly splitting cells, a system capacity can be tailored to meet traffic capacity requirements demanded by customers. For additional information on cell splitting, refer to Cellular Radio Principles and Design, by Raymond C. V. Macario, published by McGraw-Hill, Inc. in 1993, which is hereby incorporated by reference herein.
While the use of smaller cells increases a capacity of a wireless communication network, the overhead associated with implementing smaller cells is not insignificant and may even be a determinative factor in a decision by a service provider. The overhead costs associated with cell splitting are not inexpensive as cell splitting requires the addition of base stations for each of the split cells. As well, the real estate where these small cell base stations are deployed must also be obtained by the provider of wireless communication services.
In situations where the overhead costs associated with deploying additional base stations through cell splitting (using smaller cells) in new zones or land sites is prohibitive, a number of sectors within a cell of a wireless communication network can be increased. Such a configuration is illustrated in FIG. 3. A cell is divided into sectors through the use of directional antennas at base stations. The directional antennas reduce the level of interference in communications within a cell as signal interference is only detected in an area to which the directional antenna is pointed. In implementation, however, most base stations are only able to support a limited number of sectors. For example, in code division multiple access (CDMA) technology, base stations are often designed to support three sectors within a cell of a communications network. Therefore, as the number of sectors per cell is increased to meet the ever increasing demands for service, a point may soon be reach where the base station hardware is unable to support increase in a number of sectors. The use of sectors and the limitations thereof is provided in an TIA/EIA/IS-95-A and TSB74 standard published by TIA/EIA, which is hereby incorporated by reference herein.
The increase in the number of sectors requires some method of allocating the resources of the cell to the users. This point is quite critical in CDMA technologies, where frequency of operation of the sectors and the cells are the same. During handoff from one sector to another, resources are shared. Such sharing places an extra burden on the network. Therefore, a need exists for an apparatus and method for allocating hardware resources in a multi-sectored CDMA cell that implements more than one base station per cell such that increased demands for service are met.